It is well known to use extruded tube structures of polytetrafluoroethylene (PTFE) as implantable intraluminal prostheses, particularly for the vessel structures of vascular grafts. PTFE is particularly suitable as an implantable prosthesis as it exhibits superior biocompatibility. PTFE tube structures may be used for the vessel structures of vascular grafts in the replacement, repair of or supplement to a blood vessel as PTFE exhibits excellent mechanical properties and low thrombogenicity. In vascular applications, the vessel structures are manufactured from expanded polytetrafluoroethylene (ePTFE) tube structures. These tube structures have a microporous structure which allows natural tissue in-growth and cell endothelization once implanted in the vascular system. This contributes to long-term healing and patency of the graft. Vessel structures formed of ePTFE have a fibrous state which is defined by the interspaced nodes interconnected by elongated fibrils. Vessel structures formed of ePTFE having very small transverse dimensions, such as outer and inner diameters and wall thicknesses, are particularly well-suited for certain applications, such as the implantation in blood vessels, or replacement thereof, in humans.
The vessel structures of vascular grafts are frequently advantageously assembled with other vessel structures or stents. Such assemblies may provide for a vessel structure to be within another vessel structure or stent, or for the stent to be within the vessel structure. In such assemblies, it is typically preferable for the inner transverse dimension of the outer structure, such as the inner diameter of a vessel structure, to be generally the same as or slightly larger than the outer transverse dimension of the inner structure, such as a stent. Such correspondence between the inner and outer dimensions of the outer and inner structures results in the inner and outer surfaces thereof contacting one another in flush relation. This facilitates a flush, tight fit between the outer and inner structures which is normally preferred where at least one of the structures is a vessel structure of a vascular graft.
Such close correspondence between the inner and outer dimensions of the outer and inner structures may be provided by holding one or more of the inner and outer surfaces which are to be contiguous to very small tolerances during fabrication. Such precision is normally difficult, particularly when one or more of the structures is a vessel structure of a vascular graft formed of ePTFE. Such difficulty is compounded when the ePTFE vessel structure has very small transverse dimensions, such as outer and inner diameters and wall thicknesses. Fabrication of ePTFE vessel structures having very small transverse dimensions is desirable, as such vessel structures are well-suited for certain applications, as described in the foregoing.